Signal transmission system



R, M` SPRAGUE 2,372,344

SIGNAL TRANSMISSION SYSTEM Filed sept. 22, 194s 2 sheets-sheet 1 March 27, 1945,

'ATTORN EY March 27, 1945. R.v M sPRAGuE SIGNAL TRANSMISSION SYSTEM Filed Sept. 22. 1943 2 Sheets-She'et 2 @use 1- f7 JPmas/E INVENTOR BY ATTORNEY Patented Mar. 27, 1945 UNiTEo "STATES PATENT Aorfvrflcrs. A

l SIGNAL ramgilslos srs'raiu I I nobel-t Msn-mie, Manhasset, N. Y., :asignar to' Press Wireless, Inc., Chicago, Ill., a corporation oi' Delaware Application September 22, 1943,' Serial No. 503,370

5 Claims.

This invention relates to facsimile transmis,- sion systems, and more especially to systems for transmission of facsimile or similar subject matter over radio channels or other channels subject to extraneous disturbances.

A principal object of the invention is to provide a method of signal transmission whereby the deleterious effects of static or other lnterjected noise, on the signal reproduction, are reduced to a minimum. Y

A feature of the invention relates to a method of electrically distorting the normal response of a facsimile converter so as to reduce thevshadc distortion inthe reproducedsubject matter, which shadedistortion arises from interjected nois conditions such as static or the like. Another feature relates to a distorting arrangementvfor converting the normalV exponential resimilar exponential source of 1 signal. variation,

intoV a linear response; and reconverting the' linear variation at the receiver back to an exponential variation.

A further feature relates to the novel organiza.. lion, arrangement and relative proportioning of will be translated by the photoelect'ric cell into a logarithmic or exponential electric response.

vsponse of a facsimile transmitting converter or j or distortionless amplifiers for amplifying the exponential in character.

It is well-known that in analyzing .the shade variations along successive elemental -stripsof a facsimile subject-matter, vthere are many regions where, to the normal human vision, there 'is apparently a gradual andunform change of shade density between white and black. This may be termed visual shade uniformity. It is also known that the photoelectric cell response in scanning such a visual uniform region, is not linearbut is of a non-linearor exponential character. of shade density, along a given elemental strip of the subject-matter, the electric response .is

This relation is illustrated graphically in Fig. 1 where the abscissae represent variations of shade density from white to black; and the ordinates represent the electric response of the facsimile converter.` Heretofore, it has been the standard practice to employ linear photoelectric signals from the facsimile converter.

parts which cooperate to-produce an improved signal generator at a transmitter follows a nonlinear law. Accordingly in the drawings,

In fact the entire system both at the transmitter4 and at the receiver, has usually been designed with a linear transmission characteristic in mind.

I have found that in the transmission of telcphotos via radio, the normal exponential relation between the original facsimile electric signal and the visually uniform shade densities gives rise to a number of disadvantages. One of them is'that the interjection of extraneousnoise such as static crashes, and the like, causes such noise to affect 'the facsimile reproduction ofthe darker shades to a disproportionately greater extent than the lighter shades.. Consequently, noise crashes or similar disturbances are reproduced more noticeably in the darker regions ofthe reproduced Fig. 1 is a schematicblock diagram of a facsimile system embodying the inventive concept.

Fig. l is a curve diagram showing the respms'el curve of a facsimile converter at the transmitter. Fig. 1b is a curve diagram of the characteristics of the distorting amplifier or network at the transmitter.

Fig. 1 is a curve diagram of the characteristics of the distorting network at the receiver for restoring the reproduced signal back'to its original non-linear characteristic.

Fig. 2e is a schematic of a typical distortion' Anetwork at the transmitter.

Fig. 2b is a schematic' of a typical distortion network at the receiver.

. In scanning anoriginal subject-matter, ifl the absolute (i. e. independent of normal visual. response) shade gradation is uniform or linear then the photoelectric response is likewise linear.

Since however, theY reproductionis to be a true visual reproduction lof 'the original, what appears to the eye to be a visual uniformity or linearity picture than in the lighter regionsthereof. In accordance with .the present inventiorhjthese difficulties are overcome by distorting the original exponentially varying 'electric signal prior to transmission, into a linearly varying signal. For thisl purpose the original exponentially varying signal'obtained from the scanning of a picture in the facsimile converter i, is passed through a distortion amplifier or network 2 for converting the exponential response into a linearresponse. Thus as shown in Fig. 1b, the exponential input represented by the'logarithmically spaced abscissae, results in alinear output as represented by the equally spaced ordinates. This linear response is then used to control or modulate a radio transmitter 3 lin any well-known manner. At the receiver 4 the radio signals, after suitable detection. reproduce the signals electrically with.

their linear characteristics. These linear signals f are' then passed through a redistorting amplifier or'network 5 having an exponential relation between input and output asrepresented by Fig 1.

That is, for a visually uniform variation This relation is therefore the counterpart of the original signal response illustrated by curve le.

This exponential response, after suitable ampllcation, in linear ampliilers of known construction, is' applied to the facsimile reconverter 6 in the well-known manner. I have discovered that with this distorting method prior to transmission,

spirit and scope of the invention. The invention is also applicable to any system of signal translation where an absolute uniform gradation of lntensity of a signal source e. g., light intensity,

sound intensity, etc., is not apparent to the human senses as a uniform excitation because ofthe logarithmic response 'of those senses,and wherein the signal translating device whether photoelectric cell or microphone etc., responds linearly to original absolute linear gradations of intensity.

Referring to Figs. 2a and 2b, there are shown in schematic form typical networks corresponding to 2 and 5 (Fig. 1). In Fig. 2, II is the photo cell scanning the subject matter, and I2 its associated resistor. The signal is yfed to the control grid of tube I3, which is a high gain pentode.

I4 is plate resistor of tube I3, and I! is cathode bias resistor. of tube I3 is applied to the control grid of tube I6. Tube I6 in this case, is a remote cut-oil' pentode and the voltage developed across its plate resistor I1 is used to control the frequency modulator, which inay be of the type disclosed in U. S. Patent No. 2,299,937. It will be noted that the vcathode of tube I6 is returned to a point on the vvoltage divider so that the voltage drop across resistor I4 operates tube I8 in the proper region. When the photo cell is scanning black, the voltage on the 'plate of tube I3 is such as to cause The amplified signal on the plateA the crater lamp and determinesthe minimum value to which the crater lamp ever drops. When black is to be recorded tube 24 is operated at cut-on by proper adjustment of the voltage across resistor 22. Resistor 25 is adjusted so that the current through craterl lamp 26 is correct for black. Then as the signal increases from black .to the grays to white the control grid of tube 24 becomesless negative andtube 2li draws more and more plate current.` This increase in plate currentis not linear with respect to grid voltage and by proper choice of screen voltage and location on the characteristic curve, this relationship can be made logarithmic as shown in Fig. 24. Thus, the over-all characteristic of incoming frequency versus crater lamp current will be logarithmic and the distortion introduced in the transmitter will be corrected in the recorder. Preferably, the B- voltage is regulated by a suitable voltage regulator tube 28. 1

What is claimed is:

l. The method of reducing the shade distortion effects of extraneous disturbances in' a facsimile transmission or the like, wherein the amplitude of the original signal response is nonlinear, which comprises, distorting the amplitude of said response prior to transmission to render it linear, .and redistorting the received signals to restore the original non-linear amplitude characteristic. l

- 2. The method of telephoto transmission to reduce the effects of extraneous noise in the transmission channel which comprises, distorting the original exponential amplitude response of thev exponential amplitude characteristic.

3.'In a facsimile transmission system of the n kind wherein the amplitudeiI of the original signal tube I6 to operate` in its high conducting region.

Thus, the voltage on the plate of tube I8 is low. As the photo cell is then caused to scan through the grays to the white, the voltage on the grid of tube I3 increases, causing KYthe voltage on lts plate to drop, thereby in turn bringing the lgrid of tube I6 nearer cut-off. Because of the remote characteristic of tube IB, the approach to cut-off A is not linear and by judicious choice of screen grid and plate voltages this characteristic can be made logarithmic as shown in Fig. 2. Therefore, the relation between photo cell current and tube I6, plate voltage islogarithmic.

In the receiving case in Fig. 2b, the signal from the discriminator circuit isA in the form of a D. C. potential which is proportional to the incoming frequency. This discriminator may be of the to the 'control grid of tube 24, which-is a powerpentode. The plate'current of tube 24 is passed through crater lamp 26 for exposure of the negative. Resistor 25 presents holding current for response between shade gradations and electric signals is exponential, means to convert said exponential amplitude relation into a linear relation prior to transmission, a transmission channel subject to extraneous noise interjection, a receiverA connected to said channel and having means to restorev the receivedsignals to their original exponential amplitude relation, and a facsimile reconverter to which said restored signals are applied.

4. In a systemfor transmitting facsimiles and the like, a facsimile converter, means to distort the amplitude output of said converterto one having a linear-.relation between uniform shade gradations and electric signals, means to transmit said signals over a. radio channel or the like,

means to receive and detect said signals, means \to redistort said detected signals into signa-ls having the original amplitude relation with respect to shade gradations, and a facsimile reconverter upon which said distorted signals are impressed.

5. A system according to claim li in which the f first-mentioned distorting means includes a network whse electric signal input exponentially considered, results ln a linearamplitude output; and the said redistorting means includes a network whose electric signalinput, linearly considered, results in an exponential amplitude output.

' ROBERT M. SPRAGUE. 

